MPI_WAITANY(3)					     Open MPI					    MPI_WAITANY(3)

MPI_Waitany — Waits for any specified send or receive to complete.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Waitany(int count, MPI_Request array_of_requests[],
	       int *index, MPI_Status *status)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_WAITANY(COUNT, ARRAY_OF_REQUESTS, INDEX, STATUS, IERROR)
	       INTEGER COUNT, ARRAY_OF_REQUESTS(*), INDEX
	       INTEGER STATUS(MPI_STATUS_SIZE), IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Waitany(count, array_of_requests, index, status, ierror)
	       INTEGER, INTENT(IN) :: count
	       TYPE(MPI_Request), INTENT(INOUT) :: array_of_requests(count)
	       INTEGER, INTENT(OUT) :: index
	       TYPE(MPI_Status) :: status
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • count: List length (integer).

       • array_of_requests: Array of requests (array of handles).

OUTPUT PARAMETERS
       • index: Index of handle for operation that completed (integer). In the range 0 to count-1. In Fortran, the
	 range is 1 to count.

       • status: Status object (status).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       A call to MPI_Waitany can be used to wait for the completion of one out of several requests.

       The  array_of_requests  list  may  contain null or inactive handles. If the list contains no active handles
       (list has length zero or all entries are null or inactive), then the call returns immediately with index	 =
       MPI_UNDEFINED, and an empty status.

       The  execution of MPI_Waitany(count, array_of_requests, index, status) has the same effect as the execution
       of MPI_Wait(&array_of_requests[i], status), where i is the value returned by index  (unless  the	 value	of
       index is MPI_UNDEFINED). MPI_Waitany with an array containing one active entry is equivalent to MPI_Wait <#
       mpi-wait>.

       If  your	 application  does  not	 need  to  examine  the	 status field, you can save resources by using the
       predefined constant MPI_STATUS_IGNORE as a special value for the status argument.

       Example: Client-server code (starvation can occur).

	  CALL MPI_COMM_SIZE(comm, size, ierr)
	  CALL MPI_COMM_RANK(comm, rank, ierr)
	  IF(rank > 0) THEN	    ! client code
	      DO
		 CALL MPI_ISEND(a, n, MPI_REAL, 0, tag, comm, request, ierr)
		 CALL MPI_WAIT(request, status, ierr)
	      END DO
	  ELSE	       ! rank=0: server code
		 DO i=1, size-1
		    CALL MPI_IRECV(a(1,i), n, MPI_REAL, i tag, &
			     comm, request_list(i), ierr)
		 END DO
		 DO
		    CALL MPI_WAITANY(size-1, request_list, index, status, ierr)
		    CALL DO_SERVICE(a(1,index))	 ! handle one message
		    CALL MPI_IRECV(a(1, index), n, MPI_REAL, index, tag, &
			      comm, request_list(index), ierr)
		 END DO
	  END IF

ERRORS
       Almost all MPI routines return an error value; C routines as the return result of the function and  Fortran
       routines in the last argument.

       Before  the error value is returned, the current MPI error handler associated with the communication object
       (e.g., communicator, window, file) is called.  If no communication object is associated with the MPI  call,
       then  the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When
       MPI_COMM_SELF is not initialized (i.e.,	before	MPI_Init  <#mpi-init>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#mpi-finalize>,  or  when	using the Sessions Model exclusively) the error raises the
       initial error handler. The initial error handler can  be	 changed  by  calling  MPI_Comm_set_errhandler	<#
       mpi-comm-set-errhandler>	 on  MPI_COMM_SELF  when  using the World model, or the mpi_initial_errhandler CLI
       argument	 to  mpiexec  or  info	key   to   MPI_Comm_spawn   <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.   If  no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error  handler  is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT  An  error handler that can be invoked on a communicator, window, file, or session. When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the  group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       Note that per the “Return Status” section in the “Point-to-Point Communication” chapter in the MPI Standard
       <https://www.mpi-forum.org/docs/>,   MPI	  errors  on  requests	passed	to  MPI_Waitany	 do  not  set  the
       status.MPI_ERROR field in the returned status.  The error code is  always  passed  to  the  back-end  error
       handler	and may be passed back to the caller through the return value of MPI_Waitany if the back-end error
       handler returns it.  The pre-defined MPI	 error	handler	 MPI_ERRORS_RETURN  exhibits  this  behavior,  for
       example.

       See also:

	  • MPI_Comm_set_errhandler <#mpi-comm-set-errhandler>

	  • MPI_File_set_errhandler <#mpi-file-set-errhandler>

	  • MPI_Test <#mpi-test>

	  • MPI_Testall <#mpi-testall>

	  • MPI_Testany <#mpi-testany>

	  • MPI_Testsome <#mpi-testsome>

	  • MPI_Wait <#mpi-wait>

	  • MPI_Waitall <#mpi-waitall>

	  • MPI_Waitsome <#mpi-waitsome>

	  • MPI_Win_set_errhandler <#mpi-win-set-errhandler>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					    MPI_WAITANY(3)
